Permitting unary expression in slot so that `<-1>` will work.
Updated slots
+ Now they are full expression literals which can be used anywhere
+ They can contain ranges (e.g. `<2..5>`)
+ They can be constructed as regular objects (`new Slot(expr)`)
Another large batch of updates:
+ Worked on operators to have them coerce operands
+ Fixed up issuse with static properties (particularly static symbols)
+ Nailed down slots, including iterable slots
+ Got descriptors working (read+write)
+ Other misc minor changes